Ozone that is a cause of generation of photochemical smog is generated by HC and NOx that are contained in exhaust gases of automobiles and plants causing photochemical reaction. Therefore, restraining the exhaust amounts of HC and NOx from automobiles is effective means for preventing occurrence of photochemical smog by restraining generation of ozone. Meanwhile, as the means for preventing generation of photochemical smog, it is also conceivable to purify ozone in the air directly. By not only aiming at reduction in the exhaust amounts of HC and NOx which are reactants, but also achieving purification of ozone that is a product, generation of photochemical smog can be prevented more effectively. From the viewpoint like this, in some areas including the state of California in the U.S.A., automobiles equipped with air purification devices for vehicles that can directly purify ozone in the air are in practical use. The air purification device for a vehicle is especially called a DOR (Direct Ozone Reduction) system.
As the DOR system as above, for example, Patent Literature 1 discloses the DOR system in which a metallic oxide such as manganese dioxide is supported by a vehicle component. The vehicle component is installed in the spot that is in contact with the air at a vehicle traveling time, and manganese dioxide has a function of purifying ozone contained in the air by converting the ozone into other substances such as oxygen. Consequently, according to the DOR system of Patent Literature 1, ozone in the air can be directly purified while the vehicle is traveling.